Mobile devices often comprise audio components (e.g. speakers, microphones) integrated within the device. Such integration of the audio components requires consideration of the mechanical and acoustic properties of the components that may at times conflict with the desired acoustic characteristics. One of the considerations relates to acoustic resonances when said transducers are integrated inside of a mobile device.
A speaker integration for hands free functionality should produce a sufficient sound pressure level, an extended bandwidth (especially low frequency response), a low level distortion etc. However, such speaker integration inside the device may start with disadvantages due to various reasons including magnet assembly being small, limited diaphragm area, limited diaphragm excursion etc. In addition, the number of use cases is increasing in todays' devices but in contrast mechanical dimensions are reduced therefore required air cavities associated with said speakers are forced to be reduced which influences the sound quality. The air cavities and acoustic apertures for speaker integrations become vital because speaker elements including transducer dimensions, diaphragm, voice coil, suspension, and permanent magnet cannot be optimised to improve the sound quality.
There is a well-known physic concerning the rear cavity volume of the speaker, which defines the sensitivity of the resulting speaker integration and the low frequency limit of the resulting integration. It is expressed as the larger the rear volume the lower the frequency or alternatively the larger the rear volume the higher the sensitivity. These rules came about because the volume inside the rear cavity has a stiffness associated with it, which depends on the rear cavity volume and the area of the speaker diaphragm that is compressing it. Therefore, the larger the diaphragm area the stiffer the air appears to be and the smaller the rear cavity volume the stiffer the air appears to be. In both cases more force is required to compress the air inside the rear cavity volume. The fundamental resonance of a speaker integration, which does not rely on any external electronic equalisation or feedback to extend the bass response, depends only on the mass of the driver, the combined stiffness of the air inside the rear cavity volume and the suspension of the diaphragm. The combination is stiffer than either the speaker or the rear cavity volume on its own, and therefore the resonance frequency is higher. For such integrations to produce lower frequency components, a larger rear cavity volume is required which in turn exhibits a smaller stiffness and hence a lower system resonance. However, such larger rear cavity volume has an impact on the device size therefore a suitable trade off must be considered.
It is known that the resonance frequency position is important but furthermore the shape of such resonance frequency is equally important for speaker integrations. Some speaker integrations can comprise a high quality factor (Q) which is a design parameter describing how under-damped a resonance is and further characterizes a resonator's bandwidth relative to its centre frequency. A high Q resonance is narrow band which rings at the resonance frequency. The rear cavity volume has a low compliance when the speaker is acoustically coupled with the small rear cavity volume. In these circumstances, such high Q resonances may produce an undesirable output signal at the resonance frequency unless a desired damping factor is applied which requires further design considerations. A typical frequency response of speaker integration may comprise one or more resonances and at least one of these resonances may be sharp peak comparing to the rest of the frequency response. It is understood that a suitable damping is introduced by means of an electronic circuitry, one or more signal processing algorithms and/or mechanical components such as damping cloth, foam materials etc. It is known that any of these considerations either individually or their combinations define the shape of resonances.